1. Field of the Invention
This invention relates generally to the dispersing of liquids into fluidized solids. More specifically, this invention relates to a method and apparatus for dispersing a hydrocarbon feed into a stream of fluidized catalyst particles.
2. Description of the Prior Art
There are a number of continuous cyclical processes employing fluidized solid techniques in which an at least partially liquid phase stream containing hydrocarbon compounds contacts the fluidized solids in a contacting zone and carbonaceous or other fouling materials are deposited on the solids. The solids are conveyed during the course of the cycle to another zone where foulants are removed in a rejuvenation section or, more specifically, in most cases, carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the rejuvenation section are subsequently withdrawn and reintroduced in whole or in part to the contacting zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high-boiling hydrocarbons to lighter hydrocarbons. The hydrocarbon feed is contacted in one or more reaction zones with the particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons.
It has been a long recognized objective in the FCC process to maximize the dispersal of the hydrocarbon feed into the particulate catalyst suspension. Dividing the feed into small droplets improves dispersion of the feed by increasing the interaction between the liquid and solids. Preferably, the droplet sizes become small enough to permit vaporization of the liquid before it contacts the solids. It is well known that agitation or shearing can atomize a liquid hydrocarbon feed into fine droplets which are then directed at the fluidized solid particles. A variety of methods are known for shearing such liquid streams into fine droplets. U.S. Pat. No. 3,071,540 discloses a feed injection apparatus for an FCC unit wherein a high velocity stream of gas, in this case steam, converges around a stream of oil upstream of an orifice through which the mixture of steam and oil is discharged. Initial impact of the steam with the oil stream and subsequent discharge through the orifice atomizes the liquid oil into a dispersion of fine droplets which contact a stream of coaxially flowing catalyst particles. U.S. Pat. No. 4,434,049 shows a device for injecting a fine dispersion of oil droplets into a fluidized catalyst stream wherein the oil is first discharged through an orifice onto an impact surface located within a mixing tube. The mixing tube delivers a cross flow of steam which simultaneously contacts the liquid. The combined flow of oil and steam exits the conduit through an orifice which atomizes the feed into a dispersion of fine droplets and directs the dispersion into a stream of flowing catalyst particles.
Other known methods for feed dispersion include specific injection methods. U.S. Pat. No. 4,717,467 shows a method for injecting an FCC feed into an FCC riser from a plurality of discharge points. U.S. Pat. No. 5,108,583 discloses a method wherein hydrocarbons and steam are supplied from individual headers and combined in conduits to supply a steam and hydrocarbon mixture to a distribution nozzle for injection into an FCC riser.
The processing of increasingly heavier feeds in FCC type processes and the tendency of such feeds to elevate coke production and yield undesirable products has led to new methods of contacting feeds with catalyst. Of particular recent interest have been methods of contacting FCC catalyst for very short contact periods. U.S. Pat. No. 4,985,136, the contents of which are hereby incorporated by reference, discloses an ultrashort contact time process for fluidized catalytic cracking, that contacts an FCC feed with a falling curtain of catalyst for a contact time of less than 1 second followed by a quick separation. U.S. Pat. No. 5,296,131, the contents of which are hereby incorporated by reference, discloses a similar ultrashort contact time process that uses an alternate falling catalyst curtain and separation arrangement. The ultrashort contact time system improves selectivity to gasoline while decreasing coke and dry gas production by using high activity catalyst that previously contacted the feed for a relatively short period of time. The inventions are specifically directed to zeolite catalysts having high activity.
The type of injection desired for short contact time arrangements poses special problems for the injection of the feed into the catalyst. Most desirably, the feed is injected in an array of identical feed injection streams that uniformily contact a stream of catalyst flowing in a compatible pattern. Typically, the uniform array of feed injection nozzles is arranged to provide a plurality of discrete jets that extend horizontally and shoot the feed into a thin band of catalyst that falls in a direction at least partially transverse to the flow of jets. The jet array should extend over the width of the thin band which greatly exceeds its depth. In other words, the arrangement usually creates a vertical line of catalyst that is contacted by an array of jets that extends over a horizontal line. Establishing the thin but extended band of catalyst requires equipment that usually places the band toward the center of a contacting vessel. In turn, the nozzles that create the jets must also be located close to the band of catalyst. Creating an extended array of jets at a location removed from the wall of the contacting vessel creates distribution and structural problems. The distribution problems require that equal amounts of gas and oil reach each nozzle that defines the individual jet. From the structural side, providing a reliable assembly of nozzles demands that the distributor withstand possible erosive effects of the catalyst and the vibration effects imposed by the large flowing mass of catalyst and the cyclic input from numerous pieces of equipment.